1. Field of the Invention
This invention relates generally to the field of electrical testing and specifically to a test circuit for measuring the duration that an electrical element powered by alternating current is in a certain state.
2. Description of the Related Art
Dishwashers use a motor to operate a pump supplying fluid to objects to be washed. A common type of controller used to operate the pump motor includes a plurality of contacts or switches operated by a cam wheel. Such controllers are shown, for example, in U.S. Pat. No. 3,199,525 to Jellies, U.S. Pat. No. 4,525,608 to Cushing, and U.S. Pat. No. 4,559,959 to Meyers, incorporated herein by reference. At least one of the contacts is connected in series with the motor to control current flow to the motor from an alternating current supply. When the switch is closed, the motor runs. The switch is opened periodically to interrupt power supplied to the motor, thereby stopping the motor. Interrupted operation is particularly desirable when the pump is used to alternate water flow, as shown in U.S. Pat. Nos. 4,741,353 and 5,264,043 to Milocco, incorporated herein by reference. Interrupted operation can be accomplished with an auxiliary cam wheel as shown in U.S. patent application Ser. No. 08/383,055, incorporated herein by reference, or by electronic control. To optimize performance it is desirable to measure the duration of pump interruptions, that is, the time during which the motor switch is opened.
Test circuits for determining the duration of an event are known from U.S. Pat. No. 2,494,352 to Moyer, U.S. Pat. No. 2,877,405 to Morton, U.S. Pat. No. 3,235,794 to Henderson, U.S. Pat. No. 3,790,761 to Crabtree, U.S. Pat. No. 4,389,709 to Knibiehler, U.S. Pat. No. 4,751,721 to Wissell, U.S. Pat. No. 4,890,306 to Noda, and U.S. Pat. No. 5,295,172 to Liu. Still, the need remains for a simple test circuit adapted for measuring the duration of pump interruptions as well as the duration of the state of other electrical elements supplied with AC power.